I was approved for the Barclays card at age 18 all on my own. It started out at $1200 credit, not enough to buy the Mac that I wanted so I ended up using it to buy an HP computer. Once I paid the balance off, I was able to get a credit increase to $2000. Now my limit is is over $3,000 and I don't use it because of that fact. I once used it to buy a Mac Pro and when I paid $100 one month, about $80 of that went to pay the interest so I had to make higher payments to actually pay it off within a year. It's very hard for anyone to get the 0% financing, you have to have a very high credit score.
The interest rate was high, I think to start out it was like 22% but when my limit increased to over $3,000, I called to have the rate lowered and now it's 12%.
If you can, pay cash for the computer. That's what I did with my 15" MBP last month and now I don't have a monthly payment. It's too easy when the first month comes up to want to use the money for other things and you are 19 so you probably have this compulsion to spend it when you get it.